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Have you checked the temperature of the return pipe to the boiler when the radiators aren't heating up ? If the flow and return are similar temperatures the water is finding an easier path back to the boiler. I had a similar issue with the furthest away radiators not heating up well even with...

If you have the newer 637 which I think you do as it's modulating down to 6kw it has a two speed pump and if set to auto (if memory serves correctly it's D19 to check the current setting) when running under approx 50% power output the speed of the pump reduces.

You can work out the heat output of the radiator when changing the Delta T, I think in the past I have seen a figure of approx 20% reduction in output from a differential of 10c to 20c. http://www.castironradiators.ltd.uk/blog/2014/09/10/delta-t-and-heat-output-explained/ Your boiler does look...

My boiler is condensing most of the time as weather compensation means the flow is usually in the high 40s low 50s, even at -7c outside I don't think it went into the 70s (highest I saw was 68c but the flow temp drops as it reaches the target room temperature so it may have been over 70 when it...

I think the return temp difference is normal, I have checked the flow/return temp on my 630 boiler and the return is 8-12c lower with a contact thermometer compared to D41, the flow temp is within 1-2c. If 70c is going into the radiators and 50c out they may not be big enough to heat the rooms...

Reading through the posts it seems you haven't tried the 70c flow temp yet ? Wouldn't have thought turning down the boiler will have much difference if you are still trying to heat the tank to 60c with a 62c flow temp. I have just moved over to two cylinders and had to drop the native...

You can get 230v wireless switch devices from TLC, I have a set to turn on outside lights on an out building and they work well. http://www.tlc-direct.co.uk/Main_Index/Security_Menu_Index/EasySwitch/index.html?source=adwords&kw=wireless%20switch&gclid=CJG7u5mpl8QCFfDKtAodG0IA2g

It is but the only way to get weather compensation, 2 UFH zones and a radiator circuit without forking out for the commercial controls (the boiler was retained when the system was added to so no option to change brand) Works well which is main thing.

Not quite , on my system the non-Vaillant controls only open/close the actuator for the zone, the zone with the VR81 can fire the boiler and start the pump. I set the VR81 at a normal temperature and have it in the zone with the most heat loss which means the temperature will drop in this...

My main zone is around 100 sq metres with lots of windows which obviously helps as this is always going to cool down before the smaller zone, the initial plan was just to leave the small zone on all the time but it was too hot so they fitted the controller to cap the temperature. You would...

I have a couple of zones of UFH on zone 2 with Vaillant controls, mine was wired up using a 470F for the radiator zone and a VR81 for the main UFH zone with a Honeywell controller working as effectively a TRV for the small zone in a toilet/utility room. Obviously with this setup the Honeywell...

Page 5 of the manual advises the boiler can not be used on an open vented system. http://www.acwilgar.co.uk/Boiler-Manual-PDF/Vaillant/Thermo%20compact.pdf

The 350F has a set back temperature which the system maintains when outside the timed periods check what that is set to, it is normally set around 5c lower than your timed heating temperature.

I had a very similar issue when I moved into our current house, the manual bypass valve was open too wide which caused most of the flow to go straight back to the boiler causing it to cycle. Assuming all the radiators have been sized correctly a check to confirm this is when the boiler is...

You will need to ask how the Evo system is going to work with the Vaillant controller, I don't think they will work together as to run everything with full weather compensation you are going to need Vaillant controls for the UFH Zone aswell as a vr61/2 wiring centre to run the cylinder/UFH. I...

Try 17 for the code.

When I was getting quotes only one installer knew the boiler had an internal auto bypass the other 2 I got quotes off didn't and wanted to install an external one. All were Vaillant advanced installers so I was a little surprised they didn't know.

The Vaillant 637 system boiler has an internal auto bypass valve. The Vaillant controls also start and stop the system in steps so in normal operation the auto bypass valve is not activated as it first opens the zone valve, then starts the pump and a few seconds later starts the burner...

My system doesn't have a low loss header, it is a variation of hydraulic drawing 2 & 4 in the fitting instructions linked below, it has a mixer valve and vr10 on the secondary heating circuit with zone valves on the HW and heating circuit 1...

I have a similar system with a system boiler and unvented cylinder, you will need a motorised mixer valve that connects to the VR61/2 for the UFH circuit so the system can run two separate flow temperatures on the heating circuits, you can then set the different heating curves from the 470f...